Optimise no-op PATCH ops in VC HTTP API

[michaelsproul]

Issue Addressed

Closes #4936

Proposed Changes

Optimise PATCH /lighthouse/validators/{validator_pubkey} so that it is faster when processing redundant changes. I suspect this will help VCs running thousands of keys from becoming overwhelmed when processing lots of these API queries.

Longer-term we should still think about optimising the key cache and API more for actual changes. I suspect the O(n) loop over the validators for each request is part of the problem, here:

lighthouse/validator_client/src/initialized_validators.rs

Line 782 in 441fc16

.find(|def| def.voting_public_key == *voting_public_key)

Addressing this by using a persistent map would both make the API faster, and help to resolve atomicity issues like #4984.

[michaelsproul]

When spamming requests, (h)top shows Lighthouse using 20% CPU rather than 100% after this patch. I also took some screenshots of system-wide CPU metrics, but it's a bit hard to see an improvement there, because I think the system just uses the same amount of CPU to run more spam queries 😅

• green (left) = non-idle CPU
• blue = system (kernel)
• yellow (left) = user
• green (right) = user
• blue = system (kernel)
• yellow (right) = iowait

Spam script is something like:

#!/usr/bin/env bash

while [ true ]
do
	curl -s -X PATCH --data '{"builder_proposals": false}' -H "Content-Type: application/json" -H "Authorization: Bearer api-token-0x02430fd0af4a262df8be03e2f2fad494162d1d22488720fc8254639d7371b2ff7b" "http://localhost:5062/lighthouse/validators/0x90e3b075e5261c4ba40e0d33480ae16251f9383e30e1b541bd8ddfb3b04a5e64965fb5d3cabc7ff9bf52a2b028c3b9a5"
done

[macladson]

Seems good to me and I see no reason not to do it, but I also agree that some kind of map seems like a natural solution for the long term (although I'm not sure about the implications of such a change).

[michaelsproul]

@Mergifyio queue

[mergify]

queue

✅ The pull request has been merged automatically

The pull request has been merged automatically at 88b37a1